The transalkylation and disproportionation of alkyl aromatics to produce specific single ring aromatic products of high value is an important process in the petroleum refining and petrochemical industry. Disproportionation and transalkylation are well-known reactions which allow exchange of alkyl groups between two aromatic ring compounds. Such reactions typically involve the exchange of ring methyl groups, for example, the disproportionation of toluene to produce benzene and xylenes, or the transalkylation reaction between toluene and trimethylbenzene to produce xylenes. In this regard, it is highly desirable to carry out the reactions with no yield loss of aromatic rings and with retention of the desired methyl groups. For toluene disproportionation, which produces products primarily in the C.sub.6 to C.sub.8 range, certain catalysts can be utilized (such as ZSM-5 zeolite) without the use of a metal co-catalyst or added hydrogen. For transalkylation reactions involving aromatic molecules in the range C.sub.9 and higher, zeolites, having a larger pore structure than zeolite ZSM-5, are utilized and it is generally necessary to add a metal co-catalyst and operate with a co-feed of hydrogen in order to retard catalyst deactivation and to maintain an acceptable catalyst life.
Recently, there has been increasing interest in developing methods to reduce the benzene content in gasoline. As a result, there is interest in carrying out transalkylation reactions of alkyl-substituted aromatics with benzene, especially reactions of C.sub.9 and (optionally, C.sub.10 ) aromatic feedstocks, to produce valuable xylene and toluene. Even more desirable, would be a process which could produce high yields of xylene and toluene products using feedstocks containing high proportions of C.sub.9 components, including even 100 percent C.sub.9 -based feedstocks. Transalkylation of such C.sub.9 -rich feedstocks has, until now, been uneconomic due to the rapid deactivation of catalysts when feeds predominantly comprising C.sub.9 aromatics are used. It is believed that this is in large measure due to the concentration of ethyl and higher alkyl side chain groups on the aromatic ring, specifically, the ethyl toluene components in the C.sub.9 feed. These groups are much more likely to participate in condensation reactions leading to polynuclear aromatic structures which are coke precursors. In addition, transalkylation of C.sub.9 feedstocks has typically resulted in substantial levels of C.sub.10.sup.+ co-products along with the desired xylenes. It is highly advantageous to produce substantial quantities of xylene and toluene from high C.sub.9 -content feedstocks while maintaining the concentration of undesired C.sub.10.sup.+ products at a low level. The process of the present invention employing a moderately dealuminated, palladium-loaded mordenite catalyst is designed to meet this requirement.
While the use of palladium-loaded mordenite catalysts, in a variety of hydrocarbon processing reactions including transalkylation, is known in the art, there has been no suggestion or teaching of the application of a moderately dealuminated, palladium-loaded mordenite catalyst to selectively hydrodealkylate ethyl and higher alkyl groups from an aromatic ring under transalkylation reaction conditions, such as is contemplated by the present invention. For example, U.S. Pat. No. 4,489,216, assigned to Lewis, describes a catalyst for isomerization of alkenes and also disproportionation and transalkylation of aromatics. This patent describes use of a mordenite having Si/Al atomic ratio of 5-10, loaded with palladium to a level typically two weight percent. The key aspect of this disclosure is a preferential calcination of this catalyst at temperatures of 1,200-1,500 F., prior to use, to substantially remove Bronsted acid sites and maximize Lewis acid sites. This treatment is claimed to be important for alkene isomerization activity; and there is no discussion of the effect, if any, of the calcination on transalkylation activity of the catalyst much less its effect on activity of the catalyst in the hydrodealkylation of ethyl groups in C.sub.9 aromatic feedstocks under transalkylation reaction conditions. Further, the effect of moderate dealumination of the mordenite on the performance of the catalyst in transalkylation of C.sub.9 aromatic feeds containing high amounts of ethyl being substituents is clearly not disclosed.
A patent assigned to Novansky and Judec (Czechoslovak Patent No. CS 235566) describes a catalyst comprising 40% mordenite, 60% alumina and 0.5% palladium for the transalkylation of methylbenzenes, hydrodealkylation of higher alkylbenzenes, and hydro-cracking of non-aromatics. Although this patent describes the potential for hydrodealkylating higher aromatic compounds, for example, hydrodealkylation of ethyl toluene to produce toluene, which can then react with trimethylbenzenes to produce xylenes; there appears to be no recognition regarding the importance of preparing a moderately dealuminated mordenite prior to loading the palladium metal into the zeolite. Attempts to prepare and test the catalyst of this patent have not produced a catalyst which is sufficiently active, selective and stable in the hydrodealkylation of ethyl groups in C.sub.9 aromatic feedstocks to be of practical interest. Indeed, in the patent itself, there is no discussion of long-term catalyst performance in any of the patent examples.
Finally, a paper by Bawa et. al. (Erdol und Kohle, Erdgas, Petrochem, 1993, 461!:11) describes the use of a catalyst comprising 0.1% Pd on H mordenite for the transalkylation of toluene with trimethylbenzenes. However, there is no discussion of transalkylation of toluene with commercial C.sub.9 feedstocks, which include ethyltoluenes, and therefore; no recognition of the importance of hydrodeethylation in such transalkylation reactions. There is also no discussion of the importance of moderate dealumination in improving the performance of the catalyst for transalkylation reactions.
Accordingly, it would be of significant benefit to the art if a catalytic transalkylation process could be developed which would afford high yields of toluene and/or xylenes in a stable operation (minimal coking and deactivation of catalyst) with concomitant low hydrogen consumption, while allowing the use of aromatic feedstocks which contain high amounts of C.sub.9 aromatic feed components that are ring substituted with ethyl or higher alkyl groups. With such a process, the range of aromatic feed materials used to produce higher value toluene and/or xylenes could be substantially broadened beyond that heretofore possible.